class Solution {
public:
    int peakIndexInMountainArray(vector<int>& arr) {
        int left = 1;
        int right = arr.size()-2;
        //第一个和最后一个元素一定不是结果
        while(left<right)
        {
            int mid = left+(right-left)/2;
            if(arr[mid-1]<arr[mid] && arr[mid]<arr[mid+1])
            {
                left = mid+1;
            }
            else right=mid;
        }
        return left;
    }
};